Electrodes are essential for semiconductor devices. It is necessary to realize ohmic contact between an electrode and a semiconductor surface and reduce contact resistance as small as possible. For this purpose, the following two kinds of methods are usually adopted. A first method is a method of increasing impurity concentration on a semiconductor side and constructing a situation in which electrons in metal move in and out the semiconductor side through a tunnel effect. A second method is a method of selecting, as a material of the electrode, a material having a work function that can make ohmic contact with a semiconductor material surface.
However, when a conductivity type of a semiconductor crystal is an n type, it is known that, even if a metal having a work function that theoretically should make ohmic contact is selected, in many cases, the metal comes into make Schottky-contact. This phenomenon is considered to be due to so-called “Fermi level pinning”.
According to the Schottky theory, a Schottky barrier height ϕB generated on a contact surface (a junction surface) between an n-type semiconductor and metal is given by a difference (ϕM−ϕX) between a work function ϕM of the metal and electron affinity ϕX of the n-type semiconductor. However, in most cases, an energy barrier by the Schottky theory is not equal in height to an actual Schottky barrier. Such a phenomenon is called the Fermi level pinning because the phenomenon looks like an effect caused by “pinning” the Fermi level. This Fermi level pinning is a phenomenon seen at the junction with not only Si but also most semiconductors such as Ge and a metal. The unit of ϕM, ϕX, and ϕB is [V], respectively.
Contact resistivity ρC on a junction interface between an n-type semiconductor and an electrode material is in a relation of the following Expression 1 with the Schottky barrier height ϕB and donor concentration ND per unit volume of the junction interface region. Note that π in the expression is a constant.
                              ρ          C                ∝                  exp          ⁡                      (                          λ              ⁢                                                ϕ                  B                                                                      N                    D                                                                        )                                              (                  Expression          ⁢                                          ⁢          1                )            
That is, in order to form an ohmic junction interface between the n-type semiconductor and the electrode material and in order to reduce the contact resistivity ρC, either the Schottky barrier height ϕB has to be reduced or the donor concentration ND of the junction interface region has to be increased.
However, increasing the donor concentration ND of the junction interface region is limited by the solid solubility limit of donor in semiconductor in the thermal equilibrium. Usually, the donor concentration is increased to near the solid solubility limit and cannot be increased to the concentration higher than that. On the other hand, as explained above, particularly in the case of an n-type semiconductor, the Schottky barrier height ϕB cannot be sufficiently reduced to a desired level because of the Fermi level pinning phenomenon.
Furthermore, the area of the contact decreases, as the semiconductor device is miniaturized. As it is easily understood from the above Expression 1, when the contact area is represented as S, actual contact resistance Rc is ρC/S. Even when same ρC is used, the actual contact resistance Rc increases sharply with miniaturization and effectively prevents the inherent performance improvement of the semiconductor device. That is, it is strongly demanded to reduce ρC itself in order not to increase the ratio of the contact resistance to the total resistance between the drain electrode and the source electrode with the miniaturized semiconductor device.
Therefore, as explained above, it has also been attempted to achieve the ohmic contact by providing the semiconductor layer having the high donor concentration ND in the junction interface region between the n-type semiconductor and the electrode material (see Patent Literature 1: Japanese Patent Application Laid-Open No. 2012-124483 and Patent Literature 2: Japanese Patent Application Laid-Open No. 2014-41987).
For example, Patent Literature 2 states that it is known that a barrier height ϕB is generated against electrons flowing in a direction from metal to n-type Ge by the Fermi level pinning phenomenon between the n-type Ge and the metal electrode and, as a result, the contact resistance is increased, and it is assumed that, if an n-type Ge layer with an increased electron concentration (carrier concentration) is inserted between the n-type Ge and the metal electrode, a depletion layer is extremely narrowed, electrons can tunnel, and cause ohmic contact. Patent Literature 2 discloses an invention of an ohmic contact structure characterized in that an n+-type Ge layer having an electron concentration of 1019 cm−3 or more and a thickness of 2 nm or more is formed between the metal layer for electrode and the n-type Ge layer for the purpose of providing an n+-type Ge semiconductor layer forming method and an ohmic contact structure for reducing contact resistance between the electrode layer and the n-type Ge layer through an inexpensive process.